What does it take to exterminate the human species?:

I hear the supposed ability of humanity to off itself being mentioned a lot; usually this implies the use of nuclear weapons. But, what exactly would it take? How many nukes does it take to kill the planet, and what kind of scenario could possibly lead to that taking place?

Interesting. I guess even without those fancy vaults, things wouldn't have been as bad as they were in Fallout.

there is no scientific basis for expecting the extinction of the human species.

That seems enough to answer my question. I had a feeling this would be the case.

So, across the span of how much longer it takes for the sun to heat up and make Earth uninhabitable- about 5 billion years if my textbook is worth anything- are there any extinction scenarios that are at all likely?

An extinction-level asteroid impact will almost certainly happen before the sun expands — several, in fact, but our ecosystem has (barely) survived such impacts in the past, so whether this would definitively end the human race is still in question.

There are biological weapon outbreaks to consider, but these would almost certainly never be nearly as effective as the one depicted in The Stand. Nanotech disasters may also become an issue in the future, but like nuclear warfare, they likely won't be quite so complete in their destructiveness as writers would like to think.

EDIT: Also, on an off-note, remember that Fallout is deliberately based on outdated science and cultural mores, so the unlikelihood of the scenario is kind of the point.

In terms of likely hood? Super flu is the most likely since it would probably spread at a geometric rate since it infects rather than expends itself with exposure. There's the asteroids mentioned above, along with various other random cosmic shit that could wipe out the planet. I don't think chemical, radiological or nuclear weapons pack the long term oomph to make an extinction event stick.

Back in university I was taught (i.e. "ignore this fact, it's probably wrong") that the minimum viable population size for humans was around fifteen hundred.

Both of these indicate that exterminating humans is a good deal tougher than it looks... a tiny village in the remote north could theoretically be enough to survive many devastating events. It also indicates that there's a possibility of proper preparation that could result in species survival in many circumstances (a couple of thousand is small enough that we could probably put a colony of that size on Mars by the end of the century).

Catastrophic climate change harsh enough to wipe out large mammals and starve crops would probably do it. If the world got too cold or too hot then its possible that for all our technology we simply wouldn't be able to eat. We might be able to survive by fishing. Humans would become clustered around areas with access to either the sea or rivers. Doom would only come if something made conditions in those rivers too harsh for fish to survive.

Also remember that that would kill off cotton, wool and most sources of fur. Unless we could still make synthetic fibres, unlikely in those conditions, we'd have trouble clothing ourselves. And nudity is as great a death sentence in overly warm conditions as it is in cold ones.

Alternatively... well a large enough object would kill us off. Viruses and other nasties have a low success rate; someone will be immune.

Viruses could only work in conjunction with many other factors. Spanish Flu wiped out a horrifying proportion of the planet at the time, but that was after WWI already softened everyone up for it - had the same virus come at the start of the war, or a few years later, it would have had nowhere near the same impact.

We actually are due another major outbreak, but the rapid increaase in quality of medical technology means that the impact will be proportionately less than such things as the Plague and Spanish Flu, even when it does turn up (Sars, Bird Flu, Swine Flu, etc having all proven to not be as bad as expected, relatively speaking).

And nudity is as great a death sentence in overly warm conditions as it is in cold ones.

Not true. The Human Body is built at the species level for tropical climates like in Kenya. Meaning you don't have to worry about exposure or overheat in such climates. (Though personal preference for clothes/cooler climates might be an issue...)

On the other hand, death by exposure in hot climates predominately comes from dehydration, not burning to death. Thus the only reason to need clothes in a warm climate is if you are in a windy, burning hot desert with little water to work with and you need shielding to keep from losing water long enough until you find some.

"Allah may guide their bullets, but Jesus helps those who aim down the sights."

Well, I doubt that we have the requisite firepower to completely destroy the biosphere (quickly, I mean), but we might be able to damage it to the extent that we'd die out before it recovered to the extent that we could survive off of it. Perhaps we could do something with those undersea methane pockets; as I recall the record for most damage/change to the biosphere is held by an "explosion" of some plant rapidly changing the content of the atmosphere some millions of years ago.

That was the cause of the Permian extinction. The release of the methane beds coupled with two supervolcanoes going off I believe. The combined methane and sulfur release raised the temperature by about 10 degrees, turned Britain into the Sahara...

If you took every nuke on earth and fired them all in the same moment in the most damaging manner possible, it still wouldn't be enough to wipe out humans. Would it knock us back to the middle ages? Yes. Wipe us out? Not even close, there would still probably be at least half a billion of us.

More to the point, any conditions that allowed for mammalian life in general to survive would allow humans to survive. Conditions harsh enough to kill off all larger organisms would likely do us in just as well, at least in the long term.

The Siberian Traps. That's the cause of the Permian Extinction. A giant flood basalt in area roughly the size from San Francisco to Dallas to Denver and then down into northern Mexico. Yeah, a flood basalt the rough size of the American Southwest. Most of it still survives in northern Russia directly east of the Ural Mountains and butt up against the Arctic Ocean.

Another factor believed to be caused by the Siberian flood basalts were the largest coal seam fires in geologic history. Basically at the onset of the flood basalt's first eruptions it tunneled through large coal deposits and set them on fire. As is well documented across North America, coal seam fires burn for a long time and the smoke and ash doesn't stay underground. Combine this with the massive carbon dioxide release of the flood basalt and global temperatures would rise to where the methane hydrate aka "the ice that burns" would sublimate and vaporize into the ocean and atmosphere further spiraling the warming effect. (And then you have the convective forces of hundreds of thousands if not millions of square kilometers of hot lava working in tandem with those three things)

Be advised however, this kind of change to vaporize the methane hydrate required more carbon dioxide owing to the basalt flood than industrial Man has put out or will put out for centuries combined.

Earth's carbon dioxide levels present day are about 18 parts per million. In the Permian event just prior to the methane hydrate release it was estimated at around 32+ parts per million. We as a species couldn't put that much out in 500 years at our worst.

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